Field of the Invention
The present invention is a method for producing a delamination and crack prevention layer that can be used to enhance damage tolerance levels in laminated sandwich panels.
Description of the Prior Art
Laminated sandwich panels are characterized by a comparatively low weight, exceptional planar and bending strength and stiffness properties. However, the laminated sandwich panels often lack a through thickness tension, compression strengths and stiffness levels that are found in plates formed from conventional homogenous materials. The through-thickness stresses, often referred to as “weak direction stresses” are the transverse normal stress σzz and the transverse shear stresses σxz, σyz. These stresses are identified in the boxes displayed in FIG. 1.
Structural sandwich panels subject to mechanical or thermal loads will develop stresses in the face sheets, face sheet-to-core bond layers and in the core layer(s). These stresses can lead to failure initiation. If stress levels become sufficiently high, cracks can propagate along planes that are parallel, normal and oblique to a neutral surface of the panel. When high strength thermoset polymers (i.e. epoxies) are used as face sheet-to-core bonding material; the available fracture toughness (i.e., the ability to resist plastic deformation and crack growth) is generally insufficient at high stress levels.
This insufficiency allows cracks to propagate with repeated load cycles at a stable or possibly unstable rate. Once sufficient crack propagation has occurred, a face sheet can delaminate from the core; thereby, causing strain energy to be released; stiffness to be reduced; and the load carrying capacity to diminish.
FIG. 2 depicts an example of delamination of a carbon fiber/foam core sandwich panel 1 with a face sheet 2 separating from a foam core 3. For the same sandwich panel 1, FIG. 3 depicts a core shear failure in which the foam core 3 is fractured.
No crack arresting boundaries are present in FIG. 3. The placement of delamination and crack arresting boundaries is analogous to the operation of rip-stop fabrics; whereby tears are prevented from propagating across cells formed by the grid-like placement of rip-stop (higher strength) yarns.
In the prior art; Miller (U.S. Pat. No. 7,972,698) describes a series of continuous reinforcing fibers (fiberglass, carbon, etc.) disposed at different angles to strengthen foam cores of a sandwich panel and intended for use with vacuum bagging, resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM) and other resin infusion methods. The Miller reference requires structural continuity of the reinforcing fibers between the faces of the core (and optionally through the skins) which can produce electrical conductivity between skins. This electrical conductivity can be an undesirable characteristic for structures, especially those structures requiring EMI hardening. Also, the reinforcing fibers of the cited reference provide increased panel strength and stiffness at the expense of increased weight.
The Miller reference further requires continuity of the reinforcing fibers between skins such that during resin infusion; the reinforcements become tensioned and are then capable of resisting motions and maintaining positional alignment and uniformity within the foam core. If the core reinforcing fibers were made to be discontinuous, the fibers would become segmented and cantilevered.
Prior to the resin infusion process, the core reinforcing fibers are not rigidified and therefore would (1) lack bending stiffness to resist movement resulting from lateral pressure applied during the resin infusion process and (2) would be unable to maintain an intended alignment, positional uniformity and directional stiffness enhancements for the overall panel. Use of the cited reference results in a sandwich panel having a comparatively greater areal weight density. As such, the reference does not provide an optimal solution for sandwich panels requiring less weight.
The Miller reference further requires that multiple foam core strips, attached webs, inner skins and fiber reinforcements be positioned together and then infused with a flowable adhesive resin to rigidify the assembly during the molding process. The process requires several labor-intensive pre-assembly, non-continuous steps (cutting foam strips and webs to pre-determined lengths, wrapping the foam strips with fibrous outer layers, stitching fiber reinforcements through the rigid foam strips) and continuous assembly steps (adding inner and outer skin layers followed by an adhesive resin infusion step and a pressurization step).
Sandwich panels are highly engineered structural systems. To achieve their peak load carrying capacities and damage tolerance levels; all components of these layered systems must remain functional throughout the loading event. Otherwise, damage will develop and structural integrity will be compromised. As such, there is a need to minimize the effects of through-thickness normal stresses and both transverse and in-plane stresses. There is also a need to prevent face sheet and face sheet-to-core delamination and to provide crack arresting boundaries.